1. Field of the Invention
The invention relates to a nanocomposite solid material based on hexa- and octa-cyanaometallates.
More specifically, the invention relates to a nanocomposite solid material comprising nanoparticles of a metal coordination polymer with CN ligands comprising metal cations and hexa- and octa-cyanometallate anions, notably hexa- and octa-cyanoferrate anions, said nanoparticles being bound through an organometallic bond to an organic graft chemically attached to the interior of the pores of a porous solid support.
The present invention also relates to a method for preparing said solid material.
The present invention also relates to a method for fixing (binding) mineral pollutants contained in a solution, using said material.
The technical field of the invention may generally be defined as that of mineral fixers (binders).
2. Description of Related Art
Many mineral fixers (binders) have been used for fixing (binding) various mineral pollutants such as metal cations contained in various media and effluents from various industries and in particular from the nuclear industry.
Indeed, the nuclear industry uses for treating effluents with low or medium radioactivity, notably in decontamination and dismantling activities, purification techniques with volume reduction consisting in fixing (binding) radio-isotopes present in the solutions onto a mineral solid, either by ion exchanges, or by co-precipitation in order to concentrate, confine radioactivity in a reduced space.
The volumes presently treated are enormous and attain several tens of thousands of m3/year for France. The treated liquids are also of varying nature since the question is to both process primary cooling waters of nuclear plants and various effluents coming into contact with radio-isotopes.
As examples of radioactive effluents which require treatment, mention may thus be made of:                effluents from processing operations of used nuclear fuel,        evaporation concentrates,        effluents from ponds for storing used nuclear fuel,        all the washing and rinsing effluents such as the effluents from the rinsing and washing of installations, of monitoring laboratories, washroom facilities of controlled areas, etc.        solutions for regenerating resins etc.        
Among conventional mineral fixers (binders) applied notably in the nuclear industry, mention may be made of products based on manganese oxides of the Manox® type which are used for fixing (binding) elements present in solution, under various chemical forms, said elements for example being Ag, Sb, Ra, or Pb; and iron hydroxide used for fixing (binding) transuranic elements by co-precipitation. However, separation of cesium from aqueous effluents is difficult with these conventional mineral fixers (binders) since it has low affinity for the latter.
Now, radioactive cesium decontamination of liquid effluents is a major problem. Indeed, 137Cs which has a half-life close to 30 years and 135Cs which has a half-life of about 2.106 years, are the most abundant among uranium fission products, while 134Cs which has a half-life of about 2 years is an activation product of nuclear power stations.
Hexacyanoferrates (II) and (III) of many transition metals as for them have very strong affinity for cesium over a wide pH range and good resistance to irradiation [1] [2].
This is the reason why hexacyanoferrates, notably insoluble hexacyanoferrates, such as hexacyanoferrates (II) of Cu, Ni and Co, but also hexacyanoferrates of alkaline metals are among the most currently used mineral fixers (binders), in particular in the nuclear industry for separating, recovering and fixing (binding) metal ions. These insoluble hexacyanoferrates or those of alkaline metals are in particular used as ion exchangers for fixing (binding) the ions of radioactive alkaline metals such as cesium 137 with a long half-life from various industrial and nuclear effluents, for example from strongly acid solutions stemming from the reprocessing of irradiated fuels and from solutions already mentioned above.
Presently, insoluble hexacyanoferrates are thus involved in most of the methods for processing liquid radioactive waste by co-precipitation.
Potassium nickel hexacyanoferrate (II) (KNTFC) is the most studied ion exchanger for Cs decontamination. Extraction of cesium is accomplished by 1-for-1 ion exchange between potassium and Cs of the solution to be treated. This material is obtained industrially by co-precipitation of Ni(NO3)2 and K4Fe(CN)6 [3].
Hexacyanoferrates, if they have high selectivities, however have the essential drawback of having low stability, low mechanical strength which makes it difficult or even impossible to pack them in a column because of a reduction in the volume occupied by the fixer (binder) and possibly clogging which limits the number of passages of the solution in the column.
Hexacyanoferrates when they are prepared in powder form form mechanically unstable grains and are mechanically brittle, while in the compact massive form, their low specific surface area often leads to slow reaction kinetics which strongly limits their efficiency.
Indeed, it is generally difficult to combine a compact form with a high reaction rate.
Harjula et al., in Finland, were the first to propose the use of solid (massive) hexacyanoferrates for a decontamination method carried out in a column, at an industrial scale [4], [5]. These are solid (massive) potassium cobalt-hexacyanoferrates (II) which are applied for only treating limited volumes of solutions because of clogging.
In order to increase the mechanical strength of these hexacyanoferrate materials, in order to use them in a decontamination method carried out in a column, at least the three solutions are proposed in the prior art:                the first consists of synthesizing these materials by precipitation on a solid support such as an organic resin or a bentonite;        the second consists of precipitating particles of these materials within insoluble polymers, such as poly(vinyl acetate);        finally the third consists of precipitating particles of these materials directly within a porous inorganic support, for example of the mesoporous silica type.        
Composite materials comprising hexacyanoferrates and a solid support are thereby obtained. This solid support may be an organic support or an inorganic support.
If the interest is focused on composite materials with organic supports, reference may notably be made to the document of Harjula et al. [6], which, after having tested solid (massive) hexacyanoferrates, have proposed the use of fine particles of hexacyanoferrates mixed with an organic polymer for synthesizing inorganic/organic hybrid ion exchange resins. The description of the synthesis method is not presented.
The stability of these composites with an organic support is better, but the presence in majority of organic compounds strongly limits the possibilities of uses notably because of radiolysis and poses problems for the disposal of these materials.
In particular, the presence of organic compounds in large amounts limits the conditioning of these waste materials by a conventional route of the vitrification type because of the difficulties encountered during calcination and of the reduction in the synthesis throughput of the glass.
Further, the mineral portion always has the property of non-reversibility of the fixing (binding).
If the interest is now focused on composite materials with an inorganic, mineral support, they may be prepared by synthesis by co-precipitation within the support, by synthesis via a sol-gel route, by direct synthesis within a porous inorganic support, or via other routes.
A synthesis method by co-precipitation within the support is described in the document of Mimura et al. [7] which proposes co-precipitation via a direct route on an inorganic support for Cs decontamination with a column process. Potassium nickel hexacyanoferrate (II) (KNiFC) is synthesized within a silica gel by successive impregnation of the macropores with solutions of Ni(NO3)2 and then of K4Fe(CN)6. The KNiFC is then uniformly dispersed within the silica gel matrix and the KNiFC filling percentage is controlled by the number of impregnation cycles.
More recently, the document of Ambashta et al. [8] proposes the use of a magnetite-potassium nickel hexacyanoferrate composite for decontamination of Cs from radioactive effluents, assisted with a magnetic field. This composite is obtained by co-precipitation of KNiFC in an aqueous medium on magnetite particles. This magnetic complex according to the authors has the same properties as the conventional KNiFC but allows much easier recovery of the particles after separation of Cs, thanks to the magnetic nature of these particles.
With this type of synthesis by co-precipitation within the support, the composition of the final product is poorly controlled and its properties are not very reproducible. Indeed, the deposited amount of hexacyanoferrate is very poorly controlled by the co-precipitation since the adhesion of the mineral binder onto the inorganic support is exclusively accomplished mechanically and since the hexacyanoferrate fixer (binder) is therefore weakly bound to the support. The fixer (binder) may therefore be easily detached during the decontamination step. This synthesis also systematically applies a large amount of hexacyanoferrate, which is a nuisance for treating and conditioning the thereby generated waste.
A direct synthesis method via a sol-gel route is described in the document of Mardan et al. [9] where it is proposed to carry out precipitation of the hexacyanoferrate directly during the gelling of a silica sol. To do this, a silica sol is first gelled in the presence of a solution of K4Fe(CN)6. Next, the obtained hydrogel SiO2—K4Fe(CN)6 is mixed with a solution of Co(NO3) in acetone in order to obtain the hydrogel SiO2—KCoFC. This composite is then washed and then dried in air at 115° C. Particles of porous SiO2—KCoFC composites with a pore surface of the order of 180 m2/g with pores diameters between 0.005 and 0.01 μm and a pore volume of the order of 0.4 cm3/g are thereby obtained.
The composition of the obtained hexacyanoferrate is poorly controlled. A composite with a composition K1.69Co0.93Fe(CN)6, with a ratio of the order of 0.15 g KCoFC/g of SiO2 is obtained. This composite is tested in a model solution (1M HCl, with 10 ppm of Cs) in a batch method, and not on a column. Under these conditions, a Kd of 5.73 105 ml/g of composite is obtained.
Another example of this type of hybrid inorganic-inorganic material has been proposed more recently in documents [10] and [10bis], and then marketed. Here also, the description of the synthesis method is brief.
As earlier, it seems that this is a direct synthesis of potassium nickel-hexacyanoferrate within a zirconium hydroxide gel. According to the authors of this article, zirconium hydroxide was selected for applications to basic solutions, i.e. with a pH>12. The obtained material, called Thermoxid-35, appears as granules with a diameter from 0.4 to 1 mm, containing of the order of 33% by mass of ZrO2, 38% by mass of water and 28% by mass of potassium nickel-hexacyanoferrate.
This material has a porosity, for which the pore volume is of the order of 0.35 to 0.4 cm3/g for a pore size of the order of 6 nm. This composite was tested for adsorption of Cs at concentrations ranging from 0.01 to 2.0 mmol/L in a solution for which the pH varies between 6.2 and 9.6 and in the presence of 1 mol/L of NaCl. In every case, Kds of more than 1.0 104 cm3/g are obtained.
Like the standard synthesis by co-precipitation, the elaboration of composites by co-precipitation in situ via a sol-gel route also uses a large amount of hexacyanoferrate, which may attain up to 30%, but also a non-negligible amount of water. This may pose problems for treating and conditioning the thereby generated waste. Indeed, large amounts of water may cause release of hydrogen by radiolysis during storage.
Further, laboratory tests showed that the sorption kinetics on the Thermoxid was very slow since about 300 hours were required for attaining equilibrium.
Finally, possible vitrification of these compounds rich in hexacyanoferrates may cause evolvement of toxic hydrocyanic acid and which may promote volatilization of the thereby fixed (bound) cesium, then making the decontamination inoperative.
Direct synthesis of hexacyanoferrate within a porous inorganic support is described in documents [11], [12] and [13] of Loos Neskovic et al. who propose the use of porous silica beads covered with an anion exchange polymer on which is fixed (bound) an insoluble metal hexacyanoferrate in the form of a thin layer.
In this composite, the metal hexacyanoferrate anion is adsorbed on the polymer by electrostatic interactions.
The method used for synthesizing this composite is the following: an impregnation of a porous mineral support such as silica with a polymer solution, for example of the polyvinylimidazole or polyamine type, is first of all achieved. Next, the thereby coated support is cross-linked with a cross-linking agent such as methyl iodide. Optionally it is possible to generate cationic groups, such as ammonium, phosphonium, sulfonium groups on this polymer.
At the end of these steps, one has a solid support coated with a film of anion exchange polymer.
The following step consists of impregnating this material with an aqueous solution of alkaline metal (sodium or potassium) hexacyanoferrate (II) or (III). The fixing (binding) of the anionic portion Fe (CN)64− is thereby obtained on the cationic groups of the polymer. This fixing (binding) is accomplished by forming bonds of the electrostatic type. The following step, after washing, consists of dipping this solid support in a salt, for example copper nitrate, the metal of which corresponds to the insoluble hexacyanoferrate which is desirably obtained. The insoluble metal, for example cooper or nickel, hexacyanoferrate weight content is of the order of 3% based on the mass of the mineral support such as silica.
This material may then be packed into a column and it may be applied continuously in a method for decontaminating cesium-rich solutions, the latter being selectively fixed (bound) by the hexacyanoferrates. The mechanical stability of these materials is very good and they may be used over a wide range of pHs.
Nevertheless, the presence of an organic polymer in a large amount, since it covers the whole of the inorganic support, poses the problem of radiolysis after fixing (binding) of the cesium on the one hand and the disposal problem on the other hand.
Indeed, the problem is posed of knowing what will be the outcome of this material after extraction, since if it is sent to vitrification, the organic presence in a significant amount is a problem for conducting the present vitrification process.
The authors state that these materials may be vitrified. But, during vitrification, the presence of large amounts of polymer may generate problems, while the high applied temperatures during this vitrification step may cause volatilization of the cesium.
In other words, the method described in the documents of Loos-Neskovic et al. uses several organic compounds which are on the one hand the anion exchanger polymer and optionally a cross-linking compound. The presence in a non-negligible amount of these organic compounds is a problem for treating and conditioning the thereby generated waste. Indeed on the one hand, there may be evolvement of hydrogen by radiolysis of these compounds, and on the other hand, possible direct vitrification of these materials, if it allows removal of these polymers by decomposition, generates gas evolvement which may also carry away the Cs confined within the support.
Very recently, in the document of Chin Yuang Chang et.al [14], the use of functionalized mesoporous silica supports was proposed for inserting multilayers of potassium nickel-hexacyanoferrate (NiHCF═K2NiFe(CN)6) therein by successive adsorption of Ni2+ and of Fe(CN)64−. The functionalized silica support is a silica functionalized with propyl-ethylenediamine triacetate, (PEDTAFS). Indeed, propyl-ethylenediamine triacetate (PEDTA) may chelate Ni(II) and is thus used as an anchoring point for the growth of multilayers of NiHCF. In order to prepare this material, NiHCF is synthesized within a PEDTAFS powder by first immersing this powder in a solution of Ni(NO)3. Next, after filtration and rinsing, the powder is then immersed in a solution of K4(Fe(CN)6), and then again filtered and rinsed. These steps are repeated several times, i.e. 5 times. Cesium sorption tests are carried out batchwise. Kds of more than 106 mL/g are obtained, for a solution with about 100 ppm of Cs in the form of nitrate and in the presence of other ions, i.e. KNO3 and/or NaNO3 up to 3.0 M. But the fact that the functionalized porous silica also adsorbs Cs should also be taken into account.
The vitrification of the materials described in this document, after fixing (binding) of the pollutants such as cesium, may cause many problems notably related to the volalitilization and to the release of the pollutant such as cesium, during this vitrification step, because of the very high applied temperatures.
Other routes for synthesizing composite materials have also been studied. Thus, the document of Lin and Cui [15] describes organic-inorganic nanocomposites for the treatment of radioactive effluents. They use an electrochemical approach for synthesizing these materials consisting of a thin conducting polyaniline film and of nanoparticles of nickel hexacyanoferrates deposited on a matrix of carbon nanotubes. This material is intended to be used in a decontamination method by ion exchange assisted by electrochemistry.
These materials cannot be used in a column method and the treatment, the disposal of these materials which are very rich in carbon after fixing (binding), extraction of the pollutant, is difficult.
The document of Folch B. et al. [16] describes the synthesis of nanoparticles with a controlled size of coordination polymers with CN ligands (cyano-bridged coordination polymer nanoparticles) comprising hexa- and octa-metallate building blocks inside hybrid mesoporous silicas, more specifically hybrid mesostructured hexagonal silicas of the SBA-15 and MCM-41 type containing the —(CH2)2C5H4N functionalities.
The use of the composite material obtained for fixing (binding) cations is not described. Further, the mesoporous silicas have highly insufficient mechanical strength which prevents them from being applied in a column.
It appears that considering the foregoing, there therefore exists a need for a composite solid material fixing (binding) mineral pollutants based on hexacyanoferrates of metals, or more generally cyanometallates of metals which, notably in the case where these mineral pollutants are radioactive compounds such as cesium, may be easily treated, conditioned, stored, in a limited number of steps, after fixing (binding) of these mineral pollutants without any risk of volatilization, of release of these pollutants. There further exists a need for a material which retains these fixed (bound) immobilized mineral pollutants after their binding, and which do not again release, again salt out these immobilized mineral pollutants regardless of the treatment(s) undergone by the composite solid material at the end of the fixing (binding).
This material should further be chemically and mechanically stable so as to be able to be thereby packed in a column in order to allow continuous application.
This composite solid material binding mineral pollutants should also have excellent binding, in particular decontamination, properties.
On the other hand, it would be desirable to have a solid material fixing (binding) mineral pollutants associating good mechanical stability to a high reaction rate contrary to products in compact form, for which the low specific surface area leads to slow reaction rates.
In other words, there exists a need for a solid material fixing (binding) mineral pollutants, based on hexacyanoferrates of metals, or more generally on cyanometallates of metals which i.a. has excellent mechanical and chemical stabilities, a strong affinity or decontamination coefficient, great reactivity, as well as good selectivity and which may be easily treated after fixing (binding) of the pollutants without the latter being released or volatilized.
These properties should be obtained with a minimum amount of mineral fixer (binder) of the metal hexacyanoferrate type, in any case an amount significantly less than that of the composite mineral fixers (binders) of the prior art.
Finally, there exists a need for a material having perfectly reproducible and controlled composition and properties, and for a reliable method with which such a material may be prepared.
The goal of the present invention is therefore to provide a composite solid material binding mineral pollutants based on hexacyanoferrates of metals, or more generally on cyanometallates of metals, which does not have the drawbacks, defects, limitations and disadvantages of composite solid materials fixing (binding) mineral pollutants of the prior art, which overcomes the problems of the materials of the prior art and which i.a. meets the whole of the needs and requirements mentioned above.